Actuator assemblies for hydraulic braking systems

ABSTRACT

An actuator assembly for a vehicle hydraulic braking system incorporates at least two chambers each having an inlet for connection to a fluid pressure source and an outlet for connection to a wheel brake. The effective volume of each chamber is adapted to be varied between a minimum and a maximum value by movement of an expander piston working in a bore in communication with that chamber. The expander pistons are normally urged in directions in which the effective volumes of the chambers are at their minimum values, and means are incorporated for moving the expander pistons in opposite directions to increase the effective volumes of the chambers progressively when the deceleration of a wheel controlled by a brake supplied with fluid from at least one of the outlets exceeds a predetermined value.

United States Patent Ingram et al.

[ ACTUATOR ASSEIVIBLIES FOR HYDRAULIC BRAKING SYSTEMS [72] Inventors: Brian Ingram; David A. Harries, both of Warwickshire; Lancelot Phoenix, Birmingham, all of England [73] Assignee: Girling Limited, Birmingham, En-

gland 22 Filed: Sept. 17,1970

21 Appl. No.: 73,144

[30] Foreign Application Priority Data Sept. 17, 1969 Great Britain ..45,705/69 [52] US. Cl ..303/21 F [51] Int. Cl. ..B60t 8/06 [58] Field of Search ..303/21 R, 21 F [56] References Cited UNITED STATES PATENTS 3,556,609 9/1968 MacDuff ..303/21 Nov. 28, 1972 3,576,350 4/1971 Larsen ..303/21 F Primary Examiner-Richard E. Aegerter Attorney--Imirie and Smiley [57] ABSTRACT An actuator assembly for a vehicle hydraulic braking system incorporates at least two chambers each having an inlet for connection to a fluid pressure source and an outlet for connection to a wheel brake. The effective volume of each chamber is adapted to be varied between a minimum and a maximum value by movement of an expander piston working in a bore in communication with that chamber.

The expander pistons are normally urged in directions in which the effective volumes of the chambers are at their minimum values, and means are incorporated for moving the expander pistons in opposite directions to increase the effective volumes of the chambers progressively when the deceleration of a wheel controlled by a brake supplied with fluid from at least one of the outlets exceeds a predetermined value.

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SHEET 5 BF 7 PATENTED RUV 2 8 I972 INvENTaRs Attfs.

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sum 6 OF 7 INVENToRS lkiAN INGRAM DAVID A- HARRlEs ACTUATOR ASSEMBLIES FOR HYDRAULIC BRAKING SYSTEMS This invention relates to improvements in actuator assemblies for use in hydraulic braking systems for vehicles of the kind comprising at least one chamber having an inlet for connection to a fluid pressure source, for example a pedal operated master cylinder, and an outlet for connection to a slave cylinder of a wheel brake.

According to our invention an actuator assembly of the kind set forth for use in an hydraulic braking system for a vehicle incorporates at least two chambers each having an inlet for connection to a fluid pressure source and an outlet for connection to a slave cylinder of a wheel brake, in which the effective volume of each chamber is adapted to be varied between a normal minimum value, in which the inlet and outlet connections are both open, and a maximum value, in which the inlet connection is closed and the outlet connection is open, by movement of an expander piston working in a bore in communication with that chamber, the expander pistons being normally urged in a direction in which the effective volumes of the chambers are at their minimum values, and means being incorporated for moving the expander pistons in opposite directions to increase the efl'ective volumes of the chambers progressively when the deceleration of a wheel controlled by a brake supplied with fluid from at leas one of the outlets exceeds a predetermined value.

The expander pistons work in spaced parallel bores and the expander pistons are urged in directions in which the effective volume of each chamber which they control is at its minimum value by a single actuator in the form of a movable member subjected over opposed areas to differential fluid pressures which, when substantially equalized, are operative to move the actuator in a direction away from the expander pistons thereby permitting the pistons to be withdrawn from the chambers in directions to increase their effective volumes.

The actuator member may comprises a differential hydraulic piston working in a stepped bore and hydraulic fluid under pressure normally acts over an area of the differential piston of smaller diameter to maintain the effective volume of the chamber at its'minimum volume whereby, when hydraulic fluid at the same pressure acts on the area of the actuator piston of greater diameter, the actuator piston is retracted to enable the expander pistons to be withdrawn from the chambers thus increasing their effective volumes.

In another construction the actuator assembly may be combined with a fluid pressure operated booster incorporating a movable wall acting on the expander pistons and subjected on opposite sides to differential pneumatic pressures controlled by valve means adapted to be actuated, in use, when the deceleration of the braked wheel exceeds a predetermined value.

In one arrangement the movable wall acts on the expander pistons through a push-rod assembly incorporating an enlarged head for engagement with adjacent ends of the expander pistons.

The differential fluid pressure, to which the movable wall is subjected, may be that between atmosphere and a source of vacuum, such as the induction manifold of a vehicle or between a source of compressed air and atmosphere.

Conveniently movement of the expander pistons in the bores controls operation of a valve assembly controlling communication between the inlet and the outlet. Thus, when the expander pistons are urged in a direction in which the eflective volume of each chamber is at the minimum value the valve assembly is open but, when the expander pistons are moved in the opposite direction, the valve assembly closes to cut-off communication between the inlet and the chamber which it supplies.

The valve assembly is spring loaded in a closing direction and the spring loading is sufficient to effect closure of the valve assembly against the pressure of fluid in the chamber when the actuator member is retracted away from the chamber.

The valve assembly may be of the simple on-off type but, preferably, the valve assembly acts in two stages so that closing and re-establishing of communication between the inlet and outlet takes place gradually.

The expander pistons may of of equal diameters, or they may be of different diameters to relieve differentially the pressure supplied to slave cylinders of different capacities for actuating the same or different wheel brakes thereby ensuring that the braking efforts applied to the separate slave cylinders are still equal.

Some embodiments of our invention are illustrated in the accompanying drawings in which:

FIG. 1 is a layout of an hydraulic braking system;

FIG. 2 is a layout of a braking system similar to the system illustrated in FIG. 1 but showing some modifications;

FIG. 3 is a longitudinal section through a control valve for use in the braking system of FIG. 1 or FIG. 2;

FIG. 4 is a longitudinal section through a tandem actuator for use in the braking system of FIG. 1 or FIG. 2 incorporating a pair of expander pistons arranged in parallel and adapted to be actuated by a single actuator piston;

FIG. 5 is a longitudinal section of a further actuator similar to FIG. 5 but showing some modifications;

FIG. 6 is a layout of a further braking system incorporating a modified actuator assembly;

FIG. 7 is an axial section through a combined booster and actuator assembly incorporated in the layout of FIG. 7; 7

FIG. 8 is a piston of the combined booster and actuator assembly of FIGS. 8 drawn on an enlarged scale; and

FIG. 9 is a modification of the actuator of the assembly shown in FIG. 2.

In the layout illustrated in FIG. 1, 1 is a pedaloperated tandem master cylinder having two pressure spaces 2 and 3. The pressure space 2 is connected through pipe-lines4 and 5 to slave cylinders 6 for actuating brakes on the rear wheel of the vehicle, and through pipe-lines 7 and 8 to slave cylinders 9 for actuating brakes on the front wheels of the vehicle. The pressure space 3 is also connected through pipe-lines 10 and 11 to separate slave cylinders 12 for actuating the brakes on the front wheels of the vehicle.

When the brakes are applied the deceleration of each front wheel is sensed by an electrically inductive sensor 13, and the deceleration of each rear wheel is sensed by an electrically inductive sensor 14.

When the deceleration of one or more wheels exceeds a predetermined value the AC. output from the sensor on that wheel or wheels in question is fed to electronic control means which convert the AC. signal into a D.C. output. Specifically the output from each sensor 13 is fed to a separate electronic control module 15, the the A.C. signals from the sensors 14 are fed to a common electronic control module 16.

The system includes a supply circuit of hydraulic fluid under pressure comprising a reservoir 17 for hydraulic fluid feeding an electrically driven pump 18. The pump 18 pumps fluid under pressure to an hydraulic accumulator l9. Fluid under pressure from the hydraulic accumulator 19 is delivered through a first circuit to a pair of control valves 20, and is then returned to the reservoir 17. Simultaneously fluid under pressure from the hydraulic accumulator 19 is delivered through a second circuit to a third control valve 21 and is then returned to the reservoir 17.

Each control valve 20 is adapted to regulate the supply of fluid from the first circuit to an actuator 22 for controlling the braking effort applied to one of the front wheels of the vehicle in response to the D.C. output signal received from the control module 15 to which it is connected. The output signals from each control module 15 are responsive to the deceleration of the front wheel with which that module 15 is associated.

The control valve 21 is adapted to regulate the supply of fluid from the second circuit to an actuator 24 for controlling the braking effort applied to the rear wheels of the vehicle in response to the D.C. output signal received from the control module 16. The output signals from the control module 16 is responsive to the deceleration of the rear wheels of the vehicle.

A non-return valve 25 is located in the output side of the pump 18 to ensure that leak-back of pressure can not occur in the system, which is maintained at substantially 300p.s.i. by the pump. A relief valve 26 is fitted in the line between the pump 18 and the non-return valve 25 to protect the system from over-pressurization, any overspill being returned to the reservoir through a bypass 17.

The pump 18 is driven from the battery 28 of the vehicle, and the accumulator 19 incorporates a pressure switch 29 of which the contacts are held open when the system is fully pressurized. When the pressure of the system falls, the contacts of the switch 29 close and energize a relay 30 to start up the pump 18 until the system attains a normal working pressure when it is switched off by the contacts of the pressure switch 29 which open automatically again in response to the pressure attained in the hydraulic accumulator, 19.

The braking system shown in the layout of FIG. 2 is similar to that illustrated in FIG. 1 and corresponding reference numerals qualified by the suffix a have been used to indicate corresponding parts.

In this embodiment the hydraulic accumulator 19 has been omitted and the electrically driven pump 18a is supplied directly with fluid from the reservoir 17a. The electrically driven pump 18a is provided. with two outlet lines 31 and 32. The line 31 delivers fluid under pressure to the control valve 21a and, when the control valve 21a is open, fluid is returned from the control valve 21a to the reservoir 17a through a return line 33.

Similarly the pump 18a delivers fluid under pressure through the pipe-line 31 to the control valves 200 which are connected in series by a pipe-line 34. When the control valves 200 are open, fluid is returned from the control valves 20a to the reservoir 17a through a return line 35.

Normally the control valves 20a and 21a are closed so that hydraulic fluid under pressure is trapped in the lines 32 and 31 respectively on the downstream side of the non-return valve 25a between the valve 18a and the control valves.

Each control valve 20,20a and 21a is of the construction illustrated'in FIG. 3 of the drawings. As illustrated 36 is a housing having a stepped cylindrical throughbore 37 of which portions of the bore are separated by a partition member 38 having a central opening 39. A chamber 40 between one face of the partition member 38 and one end of the housing is formed with a radial port 41 connected to the return line to the reservoir 17, 17a. The opposite face of the partition member 38 forms an abutment forthe inner end of a cup-shaped sleeve 42 which is formed in its closed end.,.

tion of enlarged diameter. A spacer member 45 having a central opening 46 of substantial diameter is clamped between the sleeve 42 and an adjacent face of an annular member 47 of which the opposit'e'face is in abutment with the inner end of the plug 44.

The annular member 47 is provided with a central opening 48 of a diameter substantially equal to that of the opening 43 and is in communication with an axial passage 49 in the pump circuits.

A valve member in the form of a ball 50 is located in a chamber between the sleeve 42 and the annular member 47 and is defined by the opening 46. The ball 50 is adapted to engage with one of a pair of axially spaced seatings surrounding the openings 43 and 48 to prevent flow through that opening. The chamber 46 is connected by the actuator 22, 22a or 24, 24a through a radial passage 51 in the member 45 and a communicating radial port 52 in the cylinder wall.

Normally the ball 50 is held against the seating surrounding the opening 48 to cut-off communication between the high pressure circuit and the actuator 22,22a or 24, 240 by a push rod assembly 53 located in the chamber 40 and extending through the opening 43 at its inner end. The push-rod assembly 53 is urged in this direction by a pre-loaded spring 54 acting between an abutment plate 55 and the inner end of the casing 56 of a solenoid assembly 57 adapted to be energized by D.C. supply from one of the electronic control modules 15 or 16.

When the deceleration of one of the braked wheels exceeds a predetermined value, the solenoid 57 of the control valve 22, 22a or 24, 24a is energized by the D.C. supply from the module 15 or 16 associated with that wheel and the push-rod assembly 53 is withdrawn against the force in the pre-loaded spring 54. The high pressure fluid in the circuit acts on the ball 50 and urges it into engagement with the seating surrounding the opening 43 to cut-0E communication with thereservoir 17, 17a through the port 41. Simultaneously fluid under pressure passes to the corresponding actuator 22,22a or 24, 24a which operates in a manner to be described.

When the deceleration of the wheel is reduced to a value at least equal to the predetermined value, the solenoid 57 is de-energized and the pre-loaded spring 54 acts on the push rod assembly 53 to urge the ball 50 into engagement with the seating surrounding the opening 48. The high pressure fluid previously supplied to the actuator 22,22a or 24, 24a returns to the reservoir 17, 17a through the opening 43, the chamber 40 and the port 41.

The actuator 22, 22a which is controlled by the control valve 20, 20a is illustrated in FIG. 4 of the accompanying drawings. The actuator 22 comprises a body 58b in which is formed a pair of parallel bores 59b com municating with a bore of increased diameter having two stepped portions 60b and 61b of constant progressively increased diameter. Each bore 59b leads into a bore portion 62b of substantial diameter terminating at one end of the body 58b. A plug 63b having an axial inlet passage 64b for connection in the pipe-line 4, 4a leading to the rear wheel brakes is screwed in the outer end of each bore portion 62b and terminates short of the inner end of that bore portion.

The bores 59b and the bore portions 62b are separated by a shoulder 69b having a central opening, and a chamber 70b defined between each shoulder 69b and a corresponding bore 59b is connected to the slave cylinders 6,6a of the rear wheel brakes through radial ports 72b in the cylinder wall. I

A stepped piston assembly works in the stepped bore and comprises a differential actuator piston 73b working in the portions 60b and 61b of the bore. The piston is arranged such that when the outer end of the portions 74b of lesser diameter is in abutment with a step 75b between the bore portion 60b and the bores 59b, the portion 76b of greater diameter is spaced from a step 77b at the change in diameter between the portions 61b and 60b to define a chamber 78b connected through an inclined drilling 79b to the port 52 of the control valve 20, 20a. A cup-shaped piston 80b located within the chamber 78b is slidably mounted on the piston portion 74b and is normally urged into engagement with the step 77b by a spring 81b. The travel of the piston 80b relative to the portion 74b is limited by the engagement with the piston 80b of a stop ring (not shown) carried by the stepped piston portion 74b.

An expander piston 83b works in each bore 59b and is of such a length as to project into the chamber 70b when the portion 74b of the stepped piston assembly is in engagement with the step 75b. The end 84b of each expander piston 83b remote from the portion 74b is in abutment with the inner end of a pair of concentric valve stems 85b and 86b which project into the bore portion 65b the central opening in the member 69b. A clearance is provided between the outer stem 86b and the opening and a bleed hole on the stem 86b connects with a clearance between the stems 85 and 86, so that fluid from the master cylinder 1,1a can'pass to the slave cylinders 9, 12 of the front wheel brakes when the brakes are applied normally. The inner stern 85b is longer than the outer stem 86b, and each stem carries a valve head 90b and 91b respectively at its outer end. The inner ends of the value stems 85b and 86b are normally urged into engagement with the outer ends of the expander pistons 83b by concentric compression springs 92b and 93b in abutment with the inner ends of the plugs 63b.

The larger diameter portion 76b of the actuator piston 73b is formed in its outer end with an axially extending'recess or blind bore in an area less than the area 96b of the step in diameter between the portions 76b and 74b of the piston 73b. A piston 97b works the bore 95b and is urged towards the base of the recess by a spring 98b which acts as a stop. The spring 98b is located between the inner end of a cage 99b surrounding the portion of the body containing at least a part of the portion 61b of the bore and engaging with the piston 97b. An abutment plate 100bis secured to the body 58 and is held in a fixedly spaced relationship thereto by an axially extending cage or shroud 101b.

The piston 97b has an axial passage 102b leading into the recess 95b to which fluid under pressure is supplied from the hydraulic accumulator 19 or the pump 18a for applying to the piston a force in opposition to and greater than the force in the spring 98b. The valves 90b and 91b are normally held in open positions when the brake is applied by a force acting on the pistons 73b and 83b which is greater than the force in the springs 92b and 93b plus the force exerted on the expander piston 83b by the fluid pressure from the master cylinder 1,1a.

For normal operation of the system fluid under pressure is supplied from the master cylinder 1, 1a to the slave cylinders 6, 6a and 12, 12a of the wheel brakes to apply the brakes.

When the deceleration of one of the front wheels exceeds a predetermined value the solenoid 57 of the control valve 20,20a is energized as described above to deliver fluid under pressure from the accumulator 10 or pump 17a to the chamber 78b in the bore portion 61b. The pressure acts on the area 96b between the piston portion 74b and 76b to move the actuator piston 73b rearwardly away from the step b. Due to the loading in the springs 92b and 93b and the master cylinder pressure in the bore portions 62b the expander piston 83b follow this movement to increase progressively the efiective volume of the chamber 70b and permit the valve head 91b to cut off flow through the clearance between the stems 86b and the shoulders 69b and subsequently permit the valve head 90b to seat on the head 91b and cut-offflow through a clearance between the concentric valve stems 85b and 86b. Thus the supply of braking fluid from the master cylinder 1,1a to the slave cylinders of the front wheel brakes is cut-off. Thereafter further rearward movement of the actuator piston 73b and the expander pistons 83b serve to increase still further the effective volume of the chambers 70b to relieve the pressure applied to the front wheel brakes thus decreasing the rate of deceleration of the wheels. During this movement the piston b is initially held in engagement with the step 77b by the force in the spring 81b until the piston 80b is carried rearwardly by the piston 73b due to the engagement with the piston 80b of the stop rings.

When the deceleration of the wheel is reduced to the predetermined value and the solenoid 66 is de-ener- 'gized to close the control valve 20, 20a the supply of fluid from the accumulator 19 or pump 17a to the chamber 78b is cut-off and the chamber 78 is placed in communication through the control valve 20, 20a with the reservoir 17, 17a. Initially, fluid is returned rapidly to the reservoir 17, 17a until the piston 80b againengages with and seats against the step 77b. Thereafter the return of fluid is less rapid as it is accomplished by a bleed through an orifice 103b in the piston 80. Thus there is a delay until a point is reached in which the valves 90b and 91b open sequentially to permit re-application of the brakes in the normal manner and at the original pressure as described above. However, before the point of re-application referred to above is reached, in any case the brakes are re-applied progressively at an intermediate pressure less the that of the master cylinder 1, In due to the progressive insertion of the outer end of the expander pistons 83b into the chamber 70b to reduce the effective volumes of the chambers and pressurize the trapped volume of fluid contained therein.

The delay in relieving the pressure in the chamber 78b due to the provision of the orifice 1031) has the advantage that a sufficient interval of time is available for the sensors 14 to sense what is occuring during the progressive re-application of the brakes by themovement of the expander piston 83b into the chamber 76b. Specifically movement of the actuator piston in a direction to permit the valves 90b and 91b to close is faster than in the opposite direction to open the valve.

Reciprocation of the expander piston 83b may occur a number of times to affect re-application of the brakes in response to wheel acceleration or deceleration before the valve 90b and 91b are re-opened. Such reciprocation of the expander piston 83b is controlled by movement of the actuator piston 73b which in turn are dictated by operation of the control valve 20,20a regulating the supply of fluid under pressure to the chamber 78b as described above. Re-opening of the valves 90b and 91b occurs only when the braked wheel contacts a surface of a co-efficient of friction higher than the surface with which that wheel was previously in contact and capable of accepting a braking force corresponding to the fluid pressure trapped in the line between the actuator 22, 22a and the slave cylinder 9, 91a 12, 12a in accordance with the position of the expander piston 83b. Alternatively, re-opening of the valves 90b and 91b occurs when the pedal pressure applied to the master cylinder 1, 1 a is reduced by a sufficient amount.

The actuator described above is suitable for use when the brakes on wheels of a vehicle are cross-connected, that is to say when separate slave cylinders on the same brakes are supplied with fluid from different pressure spaces of a master cylinder, as for example the front wheel brakes of the layout illustrated in FIG. 1 and 2 of the drawings, and it is necessary for the pressure in each supply line to the slave cylinders of the same brake to be regulated simultaneously.

The advantage of the construction illustrated in FIG. 4 is that normal braking performance is available in the event of failure of one of the expander pistons 83b or its associated braking system. That is to say the totalforce applied by the actuator piston 73b is then applied to one expander piston area resulting in twice the output pressure being applied to its associated braking circuit.

In the embodiments described above each tandem actuator 22, 22a may each be embodied into a single the control valve 20, 20a by which it is concylinders of each pair are of equal diameters and crosssection the expander pistons 83b are of equal cross-section. However, when the slave cylinder of each pair are of different diameters, the cross-section areas of the expander pistons 831) are different to compensate for the difference in cross-section area. between the slave cylinders.

In the layout illustrated in FIG. 6, is a pedaloperated booster assisted tandem master cylinder provided with a primary pressure space 1 11 connected to a booster and actuator assembly 112, and slave cylinders 113 on front wheel brakes through a line 114, and a secondary pressure space 115 connected to slave cylinders 116 on the front wheels of a vehicle through a line 1 17 and to slave cylinders 1 18 on the rear wheelsof the vehicle through the assembly 1 12 and lines 1 19 and 120. The assembly 112 is also connected to slave cylinders 123 on the rear wheel brakes through a pipe-line 124. Operation of the assembly 112 is controlled by an electronic control module 121 which emits D.C. signals from A.C. signals generated by electronic sensors 122 sensing the deceleration of the rear wheels of the vehicle.

The assembly 112 is shown in detail in FIGS. 7 and 9 and comprises a booster housing in which is located a rigid movable wall 131 sealed to the housing at its peripheral edge by means of a diaphragm 132. The wall 131 carries a rearwardly extending axial rod 133 which works through abore in a partition 134 between the housing 130 and a cylindrical bore 135 in which works a piston 136 carried on the rear end of the rod 133. The piston 136 engages with the outer ends of a pair of spaced pistons 137 of relatively small diameter, each of which works in a separate longitudinally extending portion of smaller diameter in a through bore 138 of stepped form in a housing 139. The axes of the bore 138 are parallel with the axis of the bore 135. The outer ends of greater diameters of the stepped bores 138 are closed by plugs 140, and portions of the bore 138 between the inner ends of the pistons 137 and the plugs 140 defined chambers 141 of which the effective volumes are variable upon movement of the pistons 137 in the bores 138.

An annular shoulder 142 at the step in diameter of each bore 138 defines a seating for a head 143 of a valve member 144 located in the larger diameter portion of each bore 138. The head 143 is carried by a stern 145 of a diameter less than the internal diameter of the shoulder 143, the stem 145 projecting through the shoulder 142 into the smaller diameter portion of the bore 138.

A stern 147 carrying a head 148 projects through a central bore in the valve member 144 and the head is normally urged into engagement with a seating on the head 143 by a spring 149. In this position the stem 147 projects at its free end into the bore portion of smaller diameter beyond the free end of the stern 145 and the head143 is held in engagement with the seating on the shoulder 142 by a spring 150. There is a clearance between the stems 145 and 147 communicating with the bore 138 through one radial port 151.

The movable wall 131 is normally urged bya spring 152 in a direction in which the pistons 137 are advanced in their bores and engage at inner ends of reduced diameter l53'which are provided with open ended transverse slots 154, against the shoulder 142 to hold the valves 144 and 148 in open position. In this position chambers 156 and 157 on opposite sides of the movable wall 131 are each subjected to vacuum through a pipe-line 158 from the manifold of the engine of the vehicle. A double-acting valve member 159 actuated by a solenoid 165, in response to signals from the control module 121, is adapted to engage alternatively with a pair of axially spaced seatings 161 and 162. Normally the valve member 159 engages with the seating 162 to prevent atmospheric air from entering the chamber 157 through an air inlet pipe 163. Thus the booster 130 is held in a vacuum-suspended balanced condition.

The assembly 112 is connected to the slave cylinders as shown in FIGS. 6 and 7 of the drawings so that when the master cylinder 110 is operated, fluid under pressure is supplied at least to the rear wheel brakes through the valves 144 and 148 and the chambers 141.

When the deceleration of the rear wheel exceeds a predetermined value, the solenoid 165 is energized by signals received from the control module and the valve member 159 is moved into engagement with the seating 161, as shown in FIG. 8 of the drawings, to isolate the chamber 157 from vacuum and connect it to atmosphere air through the pipe 163. Thus the movable wall 131 is subjected to a pressure differential which moves the wall rearwardly against the loading of the spring 152, and permits the pistons 137 to retract in their bores 138, permitting the valves 148 and 144 to close sequentially and cut-off the supply of hydraulic fluid from the master cylinder 1 to the pipe lines 120 and 124. Further rearward movement of the pistons 137 increase the effective volume of the chambers 138 to relieve the braking effort applied to the rear wheels of the vehicle.

When the rear wheels are restored to their desired rotational speeds, the solenoid 165 is de-energized and, due to the influences of a return spring (not shown) the valve member 159 engages with its seating 162 to cut off the supply of air to the chamber 157, and place that chamber 157 again in communication with vacuum. Thus the movable wall 131 of the booster is restored to a balance condition, to advance the pistons 137 in their bores and re-apply the brakes of the rear wheels as described above.

In the modified construction illustrated in FIG. 9 where corresponding reference numerals qualified by the suffix b have been used to indicate corresponding parts, the valves 144 and 148 are replaced by a spring loaded ball valve 170 adapted to engage with the seating on the shoulder 42b. In this construction the inner ends of the pistons 137b carry axial extensions 171 of reduced diameter which extend through the seatings in the shoulders l42b and normally hold the ball 170 away from the seatings when the brakes are operating normally. When the pistons 137b are retracted in their bores 138b, the balls 70 engage with their seatings to l0 cut-ofi the supply of hydraulic fluid to the rear wheel brakes.

The operation of the construction of FIG. 10 is otherwise the same as the embodiment of FIGS. 7 and 8 and need not be described further.

In a modification of the construction illustrated in FIG. 3, the construction is as shown in FIG. 5 except that the pistons 137b and the bores 138b in which they work are of different diameters.

This has the advantage that the rate at which the pressure applied to slave cylinders of different capacities for actuating the same or different wheel brakes can be relieved, is the same.

Although we have described with reference to FIG. 6, one particular braking layout, our invention is not limited only to use in that layout, but may be connected to any particular layout of braking system to control the braking of any wheels or set of wheels of a vehicle in accordance with the deceleration of those wheels or sets of wheels, or with other brake wheels or sets of wheels of the vehicle.

We claim:

1. An actuator assembly for a vehicle hydraulic braking system comprising a housing, means in said housing defining at least two separate chambers, each chamber having a first inlet for connection to 'a fluid pressure source and an outlet for connection to a slave cylinder of a wheel brake, an expander piston working in a first bore in said housing in communication with each chamber and movable between a first advanced position, in which the effective volume of the chamber with which that bore is in communication is at a minimum and said inlet and outlet connections are open, and a second retracted position, in which the effective volume of that chamber is at a maximum and the inlet connection is closed and the outlet connection is open, a movable wall in said housing and normally acting to urge said expander pistons into said first advanced positions, said movable wall having a first area remote from said expander pistons and a second area opposed to the first area, means for normally applying fluid under pressure to said first area to urge said expander pistons into said first positions, and a second inlet connection in said housing through which fluid under pressure is adapted to be applied to said second area of the movable wall when the deceleration of a braked wheel controlled by fluid from at least one of said outlets exceeds a predetermined value, saidfluid under pressure applied to said second area equalizing the pressure over the first and second areas of the movable wall and thus permitting the expander pistons to move towards said second retracted positions.

2. An actuator assembly as claimed in claim 1, wherein said expander pistons are of equal diameters.

3. An actuator assembly as claimed in claim 1, wherein said expander pistons are of different diameters.

4. An actuator assembly as claimed in claim 1, wherein said movable wall acts on said expander pistons through a push-rod assembly incorporating an enlarged head forengagement with adjacent ends of said expander pistons.

5. An actuator assembly as claimed in claim 1, incorporating a valve assembly for controlling communication between said inlet and said outlet of each chamber sufficient to effect closure of said valve assembly against the pressure of fluid in said chamber when said actuator member is retracted away from said chamber.

"1'. An actuator assembly as claimed in claim 6,

wherein said valve assembly comprises a valve member, a seating, and a spring for normally urging said valve member into engagement with said seating.

8. An actuator assembly as claimed in claim 6, wherein said valve assembly acts in two stages whereby closing and re-establishing of communication between said inlet and outlet takes place gradually.

9. An actuator assembly as claimed in claim 1 wherein said movable wall is included in a stepped actuator piston working in a stepped bore in said housing and engaging at its inner end with said expander pistons, said first area beinglocated at its outer end of greater diameter, and said second area being defined by a shoulder at a step in diameter between portions of said stepped piston of different diameters.

10. An actuator assembly as claimed in claim 9, wherein said first area is smaller than said second area. 

1. An actuator assembly for a vehicle hydraulic braking system comprising a housing, means in said housing defining at least two separate chambers, each chamber having a first inlet for connection to a fluid pressure source and an outlet for connection to a slave cylinder of a wheel brake, an expander piston working in a first bore in said housing in communication with each chamber and movable between a first advanced position, in which the effective volume of the chamber with which that bore is in communication is at a minimum and said inlet and outlet connections are open, and a second retracted position, in which the effective volume of that chamber is at a maximum and the inlet connection is closed and the outlet connection is open, a movable wall in said housing and normally acting to urge said expander pistons into said first advanced positions, said movable wall having a first area remote from said expander pistons and a second area opposed to the first area, means for normally applying fluid under pressure to said first area to urge said expander pistons into said first positions, and a second inlet connection in said housing through which fluid under pressure is adapted to be applied to said second area of the movable wall when the deceleration of a braked wheel controlled by fluid from at least one of said outlets exceeds a predetermined value, said fluid under pressure applied to said second area equalizing the pressure over the first and second areas of the movable wall and thus permitting the expander pistons to move towards said second retracted positions.
 2. An actuator assembly as claimed in claim 1, wherein said expander pistons are of equal diameters.
 3. An actuator assembly as claimed in claim 1, wherein said expander pistons are of different diameters.
 4. An actuator assembly as claimed in claim 1, wherein said movable wall acts on said expander pistons through a push-rod assembly incorporating an enlarged head for engagement with adjacent ends of said expander pistons.
 5. An actuator assembly as claimed in claim 1, incorporating a valve assembly for controlling communication between said inlet and said outlet of each chamber and actuated by movement of said expander piston communicating with that chamber whereby when said expander pistons are urged into said advanced position in which said effective volume of each chamber is at said minimum value said valve assembly is open but, when said expander pistons are moved towards said retracted positions, said valve Assemblies close to cut-off communication between each inlet and said chamber which it supplies.
 6. An actuator assembly as claimed in claim 5 wherein each valve assembly is spring loaded in a closing direction and said spring loading of each valve is sufficient to effect closure of said valve assembly against the pressure of fluid in said chamber when said actuator member is retracted away from said chamber.
 7. An actuator assembly as claimed in claim 6, wherein said valve assembly comprises a valve member, a seating, and a spring for normally urging said valve member into engagement with said seating.
 8. An actuator assembly as claimed in claim 6, wherein said valve assembly acts in two stages whereby closing and re-establishing of communication between said inlet and outlet takes place gradually.
 9. An actuator assembly as claimed in claim 1 wherein said movable wall is included in a stepped actuator piston working in a stepped bore in said housing and engaging at its inner end with said expander pistons, said first area being located at its outer end of greater diameter, and said second area being defined by a shoulder at a step in diameter between portions of said stepped piston of different diameters.
 10. An actuator assembly as claimed in claim 9, wherein said first area is smaller than said second area. 